The invention relates to an electronically controllable braking system for motor vehicles comprising a brake pressure generator unit with a master cylinder, for generating a brake pressure, a brake pedal via which an actuation force can be introduced into the brake pressure generator unit for actuating the brake pressure generator unit, an electronically controllable brake booster for either amplifying the actuation force introduced via the brake pedal for the actuation of the brake pressure generator unit or for effecting an actuation of the brake pressure generator unit, as well as sensors which sense the brake pressure generated by the brake pressure generator unit and a parameter for an electronic control unit, which is related to the amplification of the electronically controllable brake pressure booster.
From DE 40 28 290 C1 a method for shortening of the braking distance in critical driving situations is known wherein the actuation speed of the brake pedal as caused by the driver is utilized in its respective position as the sole criterion for initiating an automatic braking operation. In the electronically controllable braking system described therein, a comparison is made between the actuation speed of the brake pedal in its respective position as caused by the driver and a constant threshold value, and depending on the result of the comparison, the emergency braking operation is initiated or not.
The method described in DE 40 28 290 C1 is disadvantageous in that only the actuation speed of the brake pedal as caused by the driver is utilized as the criterion for initiating an automatic braking operation. The actuation speed of the brake pedal is obtained by the derivative of the actuation travel of the brake pedal with respect to times Due to the fact that the actuation travel includes a non-negligible lost motion caused by backlash in the actuation mechanism, the utilisation of the actuation speed of the brake pedal as the sole criterion for initiating an automatic braking operation calls for improvement. Due to the backlash, for example, an unintentional slight touching of the brake pedal might be sufficient to initiate an automatic braking operation, which, however, in this case could result in a dangerous driving situation.
From DE 41 02 496 A1 a brake pressure control device is known wherein the brake pressure generated in the hydraulic brake circuit is measured as a parameter which is related to the actuation force applied to the brake pedal, in order to block the brake circuit in which the increased brake pressure was measured if a threshold value is exceeded.
In addition, an electronically controlled brake booster is proposed in DE 44 36 297 A1, wherein a sensor designed as a pressure sensor senses at least the pressure which prevails in the working chamber of the brake booster.
The braking systems described in DE 41 02 496 A1 and DE 44 36 297 A1 indirectly utilize the actuation force applied to the brake pedal as the criterion initiating an automatic braking operation, which force is determined according to DE 41 02 496 A1 via the brake pressure generated in the hydraulic brake circuit and according to DE 44 36 297 A1 via the differential pressure which acts at the movable wall of the brake booster. While the actuation force applied to the brake pedal is a more reliable and thus a safer criterion for initiating an automatic braking operation compared to the actuation travel, there is, nevertheless, the great disadvantage that the actuation force cannot be determined during the automatic braking operation.
In the case of automatic braking operations, a distinction must be made between those which are effected independent of an actuation of the brake pedal, in order to perform, for example, a driving dynamics control operation or an antislip control operation, and those which intensify an actuation of the brake pedal, in order to perform, for example, an emergency braking operation or an intentional braking operation, respectively. Under safety aspects, it is of decisive importance that the actuation force can be determined during the automatic braking operation as well. As an example, reference is made to a driving dynamics control where an automatic braking operation is effected independent of an actuation of the brake pedal in order to improve the stability of the motor vehicle, in particular, during cornering. If, during the driving dynamics control operation, the brake pedal is actuated, the driver's request must be evaluated in order to decide whether the driving dynamics control is to be continued or to be aborted and an emergency braking operation or an intentional braking operation, respectively, is to be performed. For the evaluation of the driver's request the actuation force represents a very reliable and safe criterion.
At first sight, it therefore appears obvious to employ a force sensor in order to sense the actuation force at the brake pedal permanently, i.e. also during an automatic braking operation. There is, however, the major drawback that a force sensor which is suited for this application is commercially available only at a considerable cost expenditure. It is also disadvantageous that such a force sensor must be integrated into the brake pedal or the actuation mechanism, respectively, which entails considerable technical efforts.
In view of this, the invention is based on the object to improve an electronically controllable braking system of the initially mentioned type with respect to the above-mentioned drawbacks in order to meet a very high safety standard at a relatively low cost expenditure.